The present invention is related to RF circuit design, particularly to a partial-suspended open-line resonator for parallel-coupled line filter system.
Planar filters are particularly desired in the RF front-end of modern communication systems because of the easier fabrication and lower cost. One type of planar filter is the parallel-coupled line filter composed of a series of half-wavelength resonant conductive lines. Furthermore, the resonators are parallel-coupled and span the distance about a quarter of one wavelength.
FIG. 1 shows a top view of a conventional parallel-coupled line filter 10. For example, the filter 10 exhibiting a third-order filter response at 2 GHz comprises three open line resonators 11. The conventional parallel-coupled line resonators 11 are commonly used in relative applications due to their simple circuit structure. The structure of the open-line resonator 11 comprises an open conductive line without via connections to a ground plane and reduces the complexity of the circuit. However, the dimension of the conventional parallel-coupled line filter 10 can be rather large because the length of the resonator 11 is approximate to half of the wavelength at the fundamental mode.
The hairpin resonator is proposed to reduce the circuit size by folding the conventional open-line resonator. But it suffers the problem in the spurious responses occurred around the harmonics of the fundamental mode, especially for oscillator and amplifier applications.
The stepped impedance resonator is another approach to shrink the circuit size and adjust the frequency of spurious mode. The stepped impedance resonator utilizes alternating high impedance and low impedance transmission line sections rather than primarily reactive components. The conventional stepped impedance resonator is composed of the conductive lines of different width connected in series. Hence the stepped impedance resonator is easy to design and typically shorter than other types of resonator. However, the conductor loss of the resonator is increased as the total conductor area enlarged for different impedance. Moreover, the impedance ratio between high impedance and low impedance is limited because of the restriction on the line width in the fabrication.
Thus, it is desirable for a resonator to provide the advantages of easy design, simple structure and well resonant response.